1. Field of the Invention
Bone grafting is among one of the most frequently performed surgical procedures by surgeons challenged with reconstructing or replacing skeletal defects. Over the years, several techniques have been devised to obtain and implant autologous bone. Scientist and clinicians have sought and defined the essential elements of bone healing and have further desired to secure these elements when considering the benefits of various types of bone grafting techniques. The essential elements required for bone regeneration are osteoconduction, osteoinduction, and osteogenic cells. In this regard, autogenous bone is the gold standard for bone harvesting. Cancellous bone contains all of these elements but lacks structural integrity. Cortical bone has structural integrity but is limited in quantity. Further, clinicians have recognized the consequences of donor site morbidity and prolonged hospitalization. To circumvent some of these issues, numerous synthetic bone like products have been made available for general use. Each product attempts to exploit one or more of the three essential elements of bone regeneration described above. Although many of these products, e.g., Pro Osteon, INTERPORE, Collagraft, ZIMMER, and others are unique, they all are very expensive and continue to escape the concepts of healthcare reform: A standard of care, with standard products, at a standard cost..COPYRGT.
To define a less invasive technique for bone harvesting, percutaneous methods have been described. The recently developed techniques simply involve using a coring cylindrical device to obtain a segment of bone. David Billmire, M.D. describes this technique in his article, Use of the CORB Needle Biopsy for the Harvesting of Iliac Crest Bone Graft, PLASTIC AND RECONSTRUCTIVE SURGERY, February 1994. Billmire makes no effort to ensure the quality of the harvested bone but rather describes a power-driven counter-rotating hollow needle as cutting through bone and soft tissue. Michael Saleh describes a percutaneous technique for bone harvesting in his article, Bone Graft Harvesting: A percutaneous Technique, Journal of Bone and Joint Surgery [Br] 1991; 73-B: 867-8. The author describes using a trephine to twist and lever out a core of bone of 8 mm in size. INNOVASIVE DEVICES describes using their COR.TM. System for arthroscopic bone graft harvesting. This system describes a disposable cutter having a distal cutting tooth projected into the lumen of the Harvester. This cutting tooth ensures that all harvested osteochondral bone grafts taken will be a uniform depth. This cutting tool also serves as means for removing the harvested bone from its donor site. Further, the plunger of the COR.TM. System is used to gently disengage the harvested bone so as to maintain the overall height of the graft. This concept is absolutely essential to the successful use of the COR.TM. System as these precisely obtained samples of osteochondral bone are implanted into pre-drilled osteochondral defects within the knee. Further, a vacuum of any sort could not be used on the COR.TM. System as the vacuum would simply continue to extract water from the knee joint, thereby failing to create an effective pressure drop across the harvested bone, and loss of operative visualization.
When considering bone for grafting purposes, the recipient site must be considered as well. Failure to achieve bony union at a fracture site may be caused by several reasons. Frequently, the blood flow is inadequate in the fracture area as a consequence of local trauma during the inciting event. Thus, when considering augmentation of the healing process with bone graft, it is imperative that the grafted bone contain all of the essential elements germane to successful osseous regeneration, namely, osteoconductive elements, osteoinductive elements, and osteoprogenitor cells. All current devices used for bone grafting focus on quantity, the osteoconductive portion of the harvested bone, and not quality, the osteoinductive portion of the harvested bone. Bone is viscoelastic, see below, and includes elements that are both elastic, osteoconductive, and viscous, osteoinductive. The osteoprogenitor cells and various proteins are within this viscous fluid state. Therefore, any device used to harvest bone to promote osseous union must consider the issues of osteoconduction, osteoinduction, and osteoprogenitor cells. Regarding cancerous bone, these fluid elements are within the interstices of the bone.
2. Information Disclosure Statement
To recognize the issues at hand governing the invention described herein, a simple discussion of biomechanics, physiology, and some general physics is warranted and presented in support hereof.
Bone is a viscoelastic material, and as such, it behaves predictably along its stress strain curve when axially loaded in either tension or compression. The key word here is viscoelastic. The prefix "visco" describes the fluid component of the material being tested and the suffix "elastic" describes the recoil potential of the material being tested. The ratio of stress:strain is Young's Modulus. Clearly, a spring is fully elastic. One may place a tension force on a spring, but when the tension is released, the spring recoils to its original length. A syringe, on the other hand, with a thin hypodermic needle attached, is considered viscoelastic. In other words, the amount of deformation observed is time dependent. Simply, the deformation will remain after the tension is removed. Consider one throwing Silly Putty against the ground and observing it bounce versus letting the material sit on a counter for several hours. One should appreciate that minimal deformation occurs when the Silly Putty bounces from the floor versus sitting on a counter for several hours. The deformation is time dependent as a consequence of the internal fluid properties of the material, an amount of time is required to observe a net fluid flow. Bone behaves in a similar fashion, but has the additional property of being able to respond to a given stress by forming new bone. When bone fails to respond favorably, it fractures.
The physiologic properties of bone hinge on the fluid elements that govern bone regeneration, namely, bone morphogenic protein, various hormones, and osteoprogenitor cells. These fluid elements are integral to the physiologic function of bone and are found within the bone marrow and the circulatory system. Appreciate that there is a net flow of these elements as bone bares a daily physiologic load during normal walking. Since the circulatory system is a closed system, a net loss of these fluid elements is not observed but rather metabolic maintenance of the various cells and proteins as they age and become nonfunctional.
Bone is incompressible above or below its elastic limit, i.e., Young's Modulus. Poisson's ratio is used to describe this behavior and can be defined as follows: EQU .nu.=-(.DELTA.d/d.sub.0)/(.DELTA.l.LAMBDA..sub.0) (1),
Poisson's ratio can be thought of as a measure of how much a material thins when it is stretched, consider taffy, or how much a material bulges when it is compressed. Regarding bone, one does not necessarily observe an increase in volume when it is compressed, but rather an increase in the density as bone remodels along the lines of stress, i.e., form follows function, Wolff's Law. When bone is compressed beyond its elastic limit, it fractures, therefore, its area will increase in a direction.apprxeq.perpendicular to the line of force. The fracture observed occurs in the osteoconductive portion of bone, and a fluid flow will occur within the osteoinductive portion of bone having the osteoprogenitor cells.
When obtaining autogenous bone for grafting, one should have an appreciation for the above principles as they will govern the quality of the bone graft material obtained. Additionally, traditional open techniques involve uncontrolled morselization of the bone graft material, thereby decreasing the amount of harvested osteoconductive bone for grafting purposes. The invention described herein exploits these principles uniquely so as to obtain unequivocally the optimum quantity and quality of bone graft material. The inventor recognizes the desire of all clinicians to eliminate donor site morbidity and promote rapid osseous incorporation of the grafted material at the recipient site. Importantly, these noble objectives will be tempered by healthcare reform.